1. Technical Field
This invention relates generally to a method and apparatus for separating valuable material from unwanted material in a mixture, such as a pulp slurry.
2. Description of Related Art
In many industrial processes, flotation is used to separate valuable or desired material from unwanted material. By way of example, in this process a mixture of water, valuable material, unwanted material, chemicals and air is placed into a flotation cell. The chemicals are used to make the desired material hydrophobic and the air is used to carry the material to the surface of the flotation cell. When the hydrophobic material and the air bubbles collide, they become attached to each other. The bubble rises to the surface carrying the desired material with it.
The performance of the flotation cell is dependent on the air bubble surface area flux and air bubble size distribution in the collection zone of the cell. The air bubble surface area flux is dependent on the size of the bubbles and the air injection rate. Controlling the air bubble surface area flux has traditionally been very difficult. This is a multivariable control problem and there are no dependable real time feedback mechanisms to use for control.
Flotation processing techniques for the separation of materials are a widely utilized technology, particularly in the fields of minerals recovery, industrial waste water treatment, and paper recycling for example.
By way of example, in the case of minerals separation the mineral bearing ore may be crushed and ground to a size, typically around 100 microns, such that a high degree of liberation occurs between the ore minerals and the gangue (waste) material. In the case of copper mineral extraction as an example, the ground ore is then wet, suspended in a slurry, or ‘pulp’, and mixed with reagents such as xanthates or other reagents, which render the copper sulfide particles hydrophobic.
Froth flotation is a process widely used for separating the valuable minerals from gangue. Flotation works by taking advantage of differences in the hydrophobicity of the mineral-bearing ore particles and the waste gangue. In this process, the pulp slurry of hydrophobic particles and hydrophilic particles is introduced to a water filled tank containing surfactant/frother which is aerated, creating bubbles. The hydrophobic particles attach to the air bubbles, which rise to the surface, forming a froth. The froth is removed and the concentrate is further refined.
In standard flotation separation, air is constantly forced through the pulp slurry to create a certain ‘flux’ of air passing through the pulp. This process, while now used widely, and refined over many decades of use, has limitations:                Due to the natural dynamics of the bubbles, a mineral-bearing particle may not typically be carried to the surface on one bubble, but may have to attach to several bubbles to reach the froth layer.        Larger particles containing minerals may not be lifted due to the limited buoyancy of a bubble, and the attractive forces between the bubble and the ore particle (created by the collector/hydrophobic chemical additives)        
In general, 10% to 15% of the mineral bearing ore in the pulp is not recovered using air-based flotation processes, and consequently, new separation technologies are being explored and developed.
The assignee of the present patent application has previously disclosed the use of polymer shells (aka “engineered bubbles”) and polymer plates as a mineral separation method, consistent with that set forth in the patent applications above. In these approaches, a polymer material is modified with an appropriate mineral collector chemistry to make the surface of the polymer attractive to the mineral of interest. In the process, minerals attach to the polymer—in the case of the shells, separation is achieved via flotation, whereas in the case of plates, it is envisioned that the bound mineral can be washed off (with the release being chemically triggered—e.g., pH for example), or mechanically released (e.g., vibration/ultrasonically for example).
Expandable polymer shells are widely used in a range of commercial products such as adhesives and sealants for example. One such example is the Dualite brand from a company named, Henkel Corp. In this case, polymer shells made of a thermoset polymer can be expanded at a particular temperature trigger point. The expansion forces come from a ‘blowing agent’—a low boiling point liquid. At the point of the liquid—vapor transition temperature, the pressure inside the microsphere increases and the sphere is ‘blown up’. Spheres can increase their size by 50 fold in volume. Their use in sealants and adhesives provides a very low density filler/foam-type expansion material.
There is a need in the industry to provide a better way to separate valuable material from unwanted material, e.g., including in such a flotation cell, so as to eliminate problems associated with using air bubbles in such a separation process.
For example, a new approach to help solve this problem may be through the use of the expandable polymer shell technology. This expandable polymer shell technology may be adapted for other uses, such as for flotation separation consistent with that set forth herein.